A cogeneration system using fuel cells of a high power generation efficiency receives attention as a decentralized generating set that can effectively utilize energy, and most fuel cells such as a commercialized phosphoric acid type fuel cell and a polymer type fuel cell under development generate electricity using hydrogen as fuel.
Currently, fuel infrastructures for hydrogen are not established, and so hydrogen needs to be generated at the installation sites. Accordingly, the steam reforming process or autothermal process, wherein hydrocarbon components such as natural gas and LPG, alcohols including methanol, or naphtha components, etc. as raw materials are allowed to react with water to generate hydrogen at a reforming section with a reforming catalyst, is utilized as a hydrogen generating method.
In such a reforming reaction of water with the raw materials, carbon monoxide is formed as a by product. For a polymer type fuel cell, which operates at low temperature, carbon monoxide acts as a poisoning component for the electrode catalyst of the fuel cell, and therefore a shifter for accomplishing shift reaction of water and carbon monoxide to hydrogen and carbon dioxide is employed along with a cleanup section for oxidizing carbon monoxide or converting it into methane.
Normally, the shifter uses both a Fe—Cr based catalyst and a Cu—Zn based catalyst. Since the Fe—Cr based catalyst is used at a comparatively high temperature (300° C. to 500° C.), carbon monoxide cannot be decreased to a large extent. On the other hand, the Cu—Zn based catalyst can decrease carbon monoxide to a considerably low concentration because it is used at a relatively low temperature (200° C. to 300° C.).
Thus, the shifter lowers the concentration of carbon monoxide to about 0.5% using a Cu—Zn based catalyst; the cleanup section uses a catalyst based on Pt or Ru, a platinum series noble metal to selectively oxidize carbon monoxide or convert it into methane and finally lowers the concentration of carbon monoxide to about 20 ppm (Certainly, the shifter should stably reduce the amount of carbon monoxide in order to reduce its quantity effectively in the cleanup section).
Nevertheless, the shifting catalyst of the Cu—Zn system is active in the shift reaction in a reduced state, and during the continuous operation of the equipment, the catalyst always remains in a reduced state, resulting in almost no degradation of the catalyst activity. However, in an intermittent operation, start-up and stop are repeated, so the shifter comes to contain air, which oxidizes the catalyst leading to an extensive reduction in the catalyst activity.
Hence, for applications wherein the start-up and stop operation are repeated frequently, when a hydrogen generating device with a Cu—Zn based catalyst is used in the shifter, carbon monoxide cannot be decreased sufficiently due to the oxidation of the shifting catalyst. When, for instance, the catalyst is used at an elevated temperature of 300° C. or more the catalyst activity is also degraded (Accordingly, the CO cleanup efficiency is also decreased).
In addition, in Japanese Patent Application No. 11-115101, a catalyst prepared by incorporating a metal of the platinum series noble metals into a metal oxide is used as a shifting catalyst in order to improve both oxidation resistance and heat resistance. The catalyst prepared by incorporating the metal of the platinum series noble metals into a metal oxide exhibits excellent features; the catalyst scarcely causes aggregation due to sintering of the catalyst species or does not change its activity in the oxidized state even if it is used at a temperature of about 500° C. However, the catalyst slightly declines in shift reactivity at low temperature as compared with the Cu—Zn based catalyst, which leads to an increase in the concentration of the carbon monoxide at the outlet of the shifter.